1. Field of the Invention
This invention relates to the preparation of certain indole naphthalein indicator dyes, to intermediates useful in the preparation of such dyes and to a method of synthesizing the intermediates.
2. Description of the Prior Art
Dyes which undergo a change in spectral absorption characteristics in response to a change in pH are well known in the art and frequently are referred to as indicator or pH-sensitive dyes. Typically, these dyes change from one color to another, from colored to colorless or from colorless to colored on the passage from acidity to alkalinity or the reverse and are commonly employed in analytical chemical procedures to measure changes in pH value. Among the indicator dyes most widely used is the group derived from phthaleins.
A particularly useful method of preparing phthalein indicator dyes including indole phthalides and naphthalides and intermediates useful in the preparation thereof form the subject matter of copending U.S. patent applications Ser. Nos. 108,662 and 393,798 of Alan L. Borror filed Jan. 21, 1971 and Sept. 4, 1973, respectively. According to this method, indole phthalides and naphthalides are prepared (1) by reacting (a) an indole and (b) phthalaldehydic or naphthalaldehydic acid to form the corresponding (na)phthalidyl-substituted indole; (2) oxidizing the (na)phthalidyl-substituted indole to the corresponding oxidation product and (3) reacting the oxidation product with an indole, preferably, in the presence of an acid catalyst to yield the corresponding dye product. The expression "(na)phthalidyl" is intended to denote either the corresponding phthalidyl- or naphthalidyl-substituted indole depending upon the selection of phthalaldehydic or naphthalaldehydic acid.
The present invention is concerned with an improvement in the above method which is especially useful and convenient for producing indole naphthaleins on a commercial scale.